The present invention relates to a sensor, in particular an electrochemical sensor.
Known electrochemical sensors include an electrochemical element, which has an electrochemical pump cell having a preferably planar, first solid electrolyte body and a first and a second preferably porous electrode. Moreover, these sensors include an electrochemical sensor cell, interacting with a pump cell, having a preferably planar, second solid electrolyte body and a third and a fourth preferably porous electrode. This sensor further includes a gas access opening and a gas access channel, so that an inner hollow space, also called a gas compartment, is connected to a measuring-gas compartment. Arranged in the hollow space, which is formed by a recess in at least one of the solid electrolyte bodies, is a diffusion resistance device which can include a porous filling. Thus, the measuring gas arrives in the gas compartment via the gas access opening and the gas access channel, the first and the second electrode of the pump cell acting so as to regulate the admission of the measuring gas into the gas compartment, and thus assuring a controlled partial pressure of the gas component to be measured. The electrochemical potential difference between the third and the fourth electrode of the second solid electrolyte body arises because of the different partial gas pressures in the diffusion resistance device, as well as in a reference-gas compartment required, for example, in the second solid electrolyte body. This potential difference can be detected by a voltmeter situated outside of the electrochemical element.
It has also been suggested to cover the gas access opening with a porous covering to prevent liquid constituents which can be contained in the measuring gas (e.g. gasoline) from penetrating into the interior of the sensor, thus essentially into the gas compartment. This covering is a porous layer on the surface of the electrochemical element facing the measuring-gas compartment. The covering is penetrable by the measuring gas, but represents a barrier for liquid constituents contained in the measuring gas. The stored-up liquid, held back in the covering, evaporates quickly after a provided heating device switches on. The porous covering is arranged on the outer pump electrode and is made, for example, of ZrO2. This covering can contain platinum, and can make oxygen from the measuring gas available for the pumping. Moreover, this covering is intended, on the one hand, to prevent soiling of the outer pump electrode, and on the other hand, to form the already mentioned barrier for the liquid constituents in the measuring gas.
Nevertheless, the measuring gas, which is not greatly hindered by this protective layer, passes through the protective layer quickly, and thus arrives at the outer pump electrode. This means that, with the changing gas composition of the measuring gas, the gas atmosphere at the outer pump electrode can also change very quickly. Consequently, the vacancy concentration at the electrode, and thus the internal resistance of the pump cell, also changes. However, depending on the energy supply (current or voltage source) of the pump cell, the pump current will then also change immediately, even before the gas composition in the hollow space of the sensor has newly adjusted. Thus, the gas adjustment in the gas compartment lags behind the gas adjustment at the outer pump electrode. This interrelation causes the known, but unwanted, phenomenon of the lambda=1 ripple (the output signal manifesting counter- or overshoot-oscillation in response to an abrupt gas exchange).
Sensors of the type described above, under the technical designation of planar wideband-lambda probes, have been used, for example, in the technology of catalytic exhaust emission control of internal combustion engines.
The present invention makes available an electrochemical sensor for ascertaining a concentration of gas, e.g., a concentration of oxygen, in a measuring gas, the sensor having an electrochemical element. The sensor includes a first solid electrolyte body having an electrochemical pump cell, which has a first and a second electrode. The sensor furthermore has a gas compartment, which is connected via a gas access opening to the measuring-gas compartment, and in which one of the two electrodes is arranged. In addition, the sensor has a second solid electrolyte body having an electrochemical sensor cell (Nernst cell), which includes a third and a fourth electrode. The surface of the first solid electrolyte body facing the measuring-gas compartment and the gas access opening are covered by a porous protective layer. The electrochemical sensor of the present invention has the particular feature that a layer, which exhibits a higher density, i.e., a lower porosity compared to the protective layer, is allocated to the porous protective layer. Because a protective layer having a higher density or lower porosity is provided, the access of the measuring gas to the outer pump electrode is delayed. This has the advantage that the pump current first changes when the measuring gas has also reached the hollow space, thus the gas compartment. In this manner, the xe2x80x9clambda=1 ripplexe2x80x9d is prevented. It is thus ensured that the access of the measuring gas to the outer pump electrode does not take place substantially earlier than to the inner pump electrode, thus to the second electrode and to the third electrode.
One preferred exemplary embodiment has the feature that the layer and the protective layer exhibit the same density or porosity. Thus, a single layer is formed which quasi performs a double function. On the one hand, the protective layer prevents liquid constituents contained in the measuring gas from penetrating into the gas compartment. On the other hand, the delayed access of the measuring gas to the first electrode (outer pump electrode) is achieved. In addition, this layer has the function of preventing the ageing of electrodes caused by exhaust gas components.
Alternatively, in a further exemplary embodiment, a gas-tight covering layer, for example, a layer made of ZrO2, can be arranged on the protective layer, thus facing the measuring-gas compartment. In a preferred specific embodiment, the layer has a thickness which can amount to 20 xcexcm. In this case, the protective layer can have a lesser density than the gas-tight covering layer.
Dense-sintering zirconium dioxide is preferred as material for the structure of the layer and/or the protective layer of the sensor according to the present invention. Alternatively, it is possible to use aluminum oxide (Al2O3), which is nanocrystalline, and therefore dense-sintering.
The gas access opening can be connected to a gas access channel which is formed, at least partially, as a hollow space, and which can be filled with a porous filling. This hollow space prevents capillary passing-on of liquid such as gasoline to the inner porous filling. The hollow space can be produced by burning off sublimable material during the sintering process.
The gas access opening can be covered by a porous covering. This covering is preferably formed from a porous material that can be the continuation of the porous protective layer which overlays the surface of the electrochemical element facing the measuring-gas compartment.
According to a variant of the present invention, the porous filling usually provided in the gas compartment is omitted. Consequently, the diffusion barrier is eliminated, and the access of the measuring gas to the second and the third electrode within the gas compartment is accelerated, so that the equally rapid adjustment of the composition of the measuring gas at all three electrodes (first to third) can be achieved in this manner, as well.
Electrochemical sensors according to the present invention and their electrochemical elements are expediently manufactured by beginning with plate-like or foil-type oxygen-conducting solid electrolytes made, for example, of stabilized zirconium dioxide, and coating them on both sides with, in each case, an inner and outer pump electrode having the necessary printed circuit traces. In this context, the inner pump electrode is located advantageously in the edge area of a diffusion or gas access channel, through which the measuring gas is supplied, and which functions as the gas diffusion resistance. The pump cell obtained in this manner can then be laminated together with a sensor cell (Nernst cell), produced in a similar manner, composed of a second solid electrolyte foil and a third solid electrolyte foil, possibly constructed to form a heater unit, and be sintered, for example, at 1300 to 1550xc2x0 C.
For manufacturing the porous fillings, one begins, for example, from porously sintering foil inserts made of a ceramic material having suitable thermal expansion properties that correspond or come close to those of the solid electrolyte foils used. Advantageously used for the filling is a foil insert made of the ceramic material from which the solid electrolyte foils are also made, it being possible to produce the porosity of the insert using pore-forming materials such as thermal carbon black powder, organic plastics, or salts which burn, decompose, or evaporate during the sintering process. The starting materials are used in such quantitative proportions that porosities of 10 to 50% are yielded after sintering, the average pore diameter being approximately 5 to 50 xcexcm.
The present invention offers the advantage that the gas access from the measuring-gas compartment to the outer pump electrode is delayed compared to known devices, to the extent that the gas access to the outer pump electrode does not take place substantially earlier than to the inner pump electrode (xe2x80x9csecondxe2x80x9d) or to the Nernst electrode (xe2x80x9cthirdxe2x80x9d), but rather that the measuring gas generally reaches the second and the third electrode sooner or at the same time as the outer pump electrode (xe2x80x9cfirstxe2x80x9d). Thus, the disadvantages of the related art, which, for example, lie in the restricted control dynamics of wideband-lambda probes caused by a strong lambda ripple, are overcome.
In a particularly advantageous manner, the present invention relates to wideband-lambda probes for ascertaining the xcex value of gas mixtures in internal combustion engines. In this context, the xcex value or the xe2x80x9cair numberxe2x80x9d is defined as the relationship of the prevailing air-fuel ratio to the stoichiometric air-fuel ratio. The probes measure the oxygen content of the exhaust gas on the basis of a change in the limiting current.